The present invention relates generally to zinc-air batteries, and in particular, to a method of detecting the battery capacity in a zinc-air battery.
Zinc-air batteries are typically used to power small instruments, such as hearing-aid devices. Other small electronic instruments or devices, such as pagers or combination pager/watches utilize zinc-air batteries for power as well. In one preferred application, the zinc-air cell is placed in a sealed battery compartment. Oxygen used by the zinc-air cell to produce power is provided through a tiny air hole in the battery chamber. An example of this arrangement can be seen in U.S. application Ser. No. 08/834,133, entitled "Container for Electrical Device Utilizing a Metal Air Cell," which itself is assigned to the present assignee and incorporated by reference as if fully set forth herein.
However, zinc-air batteries in pagers require varying loads to be placed thereon since the operation of the various components like the RF receiver, the signal processor, the microcontroller or the transducer require varying amounts of current. Thus, in a pager, current draw varies which in turn causes fluctuations in the battery voltage. Unfortunately, and as will now be described, battery voltage itself is not a valid indication of the capacity left in the cell.
For example, reference is briefly made to FIG. 1 which illustrates typical zinc-air discharge curves for three different load conditions (1.0 mA, 1.5 mA and 2.0 mA). Each of the curves' characteristics demonstrate that zinc-air batteries maintain, for essentially their entire life, an extremely constant voltage. However, as the battery life comes to an end, there is a significant and dramatic fall-off of the battery voltage. Section 101 of each respective curve illustrates the voltage level that can be maintained by the respective batteries during their life, while section 102 of each curve illustrates the dramatic fall-off at the batteries' end of life.
Additionally, FIG. 2 which also illustrates typical zinc-air load curves, demonstrates that the zinc-air battery's voltage is highly dependent on the load placed thereon. In particular, FIG. 2 illustrates a typical voltage/current profile of a good battery 201 during the time in region 101 of FIG. 1 while battery 202 is illustrated near its end of life, and in particular, nearer portion 102 of the curves of FIG. 1.
Accordingly, a method of battery capacity detection that better alerts a user when a battery is near its end life so that replacement can be effectuated prior to the device being inoperable and that achieves the below mentioned objectives is desired.